Patent FR-A No. 2178176 discloses a fabrication method for a plane lightwave or optical waveguide whereby a plane glass substrate is coated by vapor deposition with a first layer of flame-hydrolyzed glass soot having a refractive index greater than that of the substrate, said glass soot being heated to sintering temperature such as to form a first outside surface coating parallel to the substrate surface, a second coat of glass soot being then deposited by flame-hydrolysis on said first outside layer to form a layer having a lower refractive index than the first layer, the two coatings, meaning the first layer and the second glass soot layer, being then heated together until sintering of the second coating.
The process just-mentioned does not provide fully satisfactory waveguides however, due to the presence of hydroxyl ions, formed during the hydrolysis reaction, in the layer intended to transmit the light signals, which cause certain significant wavelengths of the signal to be substantially absorbed, resulting in an excessive attenuation of signals per unit length of waveguide.